kdbus: make access error behaviour consistent with dbus-daemon

kdbus: make access error behaviour consistent with dbus-daemon

Bugfix. When used with dbus-daemon, when an AccessDenied or ServiceUnknown
error occurs, gdbus returns a dbus error message to the sender (synchronously
or asynchronously) and doesn't set GError. Up to now the kdbus implementation
didn't return any error message and did set GError, which made the following
low-level functions behaviour inconsistent between dbus-daemon and kdbus:

g_dbus_connection_send_message_with_reply_sync
g_dbus_connection_send_message_with_reply
g_dbus_connection_send_message_with_reply_finish

This patch makes this behaviour consistent. Any code that relied on the inconsistent
behaviour should be corrected. The high-level function interface, returning GVariants
is NOT affected by this change.

Change-Id: I60cd4822ccef0e0fc0d7b99244f4ceb03512b97f